SBruce,
Not saying that N/S cases won't work in the ARs, just that at some point you will have problems with them. It's a bit like making a mad dash across a freeway during rush hour traffic, surviving, and declaring that it's perfectly safe; it's not, and eventually, you're gonna become a bumper sticker on a Mack truck if you keep doing it often enough.
If you're lucky, you'll eventually get a round that won't fully chamber, but is jammed so tightly in the chamber that you can't extract it by simply pulling on the charging handle. This leads to setting the butt on the ground and stomping on the charging handle (yes, I've seen military armorers doing this, frantically, on the line at Perry). Not to mention the really ugly possibllities that this potentially invites, you can usually count on a bent charging handle, and some other cosmetic damage to the reciever. All in all, not a good thing.
It's foolish to learn from your own mistakes, if you can learn from the mistakes of others. Trust me, it's cheaper, less painful, less embarrassing and saves a whole lot of frustration this way. Full length on the Service Rifles always, neck sizing (if you absolutely must) only on the bolt guns.
